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l ‐THREONINE DEAMINASE IN MARINE PLANKTONIC ALGAE. V. KINETIC EVALUATION OF FEEDBACK EFFECTS FROM STRUCTURAL ANALOGS OF l ‐ISOLEUCINE ON THE DEAMINASE ACTIVITY OF SEVEN UNICELLULAR SPECIES AND ELUCIDATION OF THE ALLOSTERIC BASIS FOR ANOMALOUS FEEDBACK CONTROL SHOWN BY A DIATOM AND A CRYPTOMONAD 1, 2
Author(s) -
Antia Naval J.,
Kripps Robert S.,
Desai Indrajit D.
Publication year - 1975
Publication title -
journal of phycology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.85
H-Index - 127
eISSN - 1529-8817
pISSN - 0022-3646
DOI - 10.1111/j.1529-8817.1975.tb02749.x
Subject(s) - valine , isoleucine , biology , biochemistry , amino acid , threonine , enzyme , leucine , stereochemistry , chemistry , serine
SUMMARY The biosynthetic l ‐threonine deaminase of 2 cyanophytes, 1 rhodophyte, 2 cryptomonads, and 1 chlorophyte showed sensitive and powerful feedback inhibition from l ‐isoleucine, which effect was closely duplicated by its isosteric structural analog l ‐O‐ methylthreonine. The l ‐isoleucine inhibition was maximal in the pH range 7.0–8.5 and was rapidly weakened with more alkaline pH. Glycine, l ‐alanine, and d ‐isoleucine had no effect, while l ‐α‐amino ‐n‐ butyric acid, l ‐norvaline, l ‐leucine, and l ‐valine caused insensitive partial inhibitions detectable only at high concentration (100‐fold greater than l ‐isoleucine). Tests of these analogs at low concentration revealed the specific capacity of l ‐valine to invoke sensitive stimulation of enzyme activity from all the algal strains, and this effect was markedly enhanced by decrease in substrate concentration. Valine also counteracted the inhibitory influence of isoleucine. It was concluded from the reaction kinetics observed with these effectors that the algal threonine deaminases conform to Monad's K system of allosteric enzymes. The activity from the cryptomonad Hemiselmis virescens was exceptional in showing 2 closely spaced pH optima, which were related to marked difference in effects from isoleucine and valine at these pH values. It was inferred from various pH‐related tests that this cryptomonad may contain 2 enzymes basically similar in biosynthetic function but differing only in pH optima and sensitivity to feedback regulation. The enzyme of the diatom Cyclotella nana was unique in showing insignificant feedback reaction from l ‐isoleucine and its structural analogs, excepting l ‐valine, which caused the strongest stimulation among the algae surveyed. It was concluded from the reaction kinetics that this deaminase is also of the biosynthetic type but that its feedback‐inhibition site is modified or unavailable, presumably from having this site already filled by endogenous isoleucine from the diatom.